This invention relates to telecommunication systems of the fully digital type, and especially to arrangements in such systems for providing synchronization between the subscriber's terminal and the system node or switch to which that terminal is connected.
In such a system voice, data and signaling information are transmitted in a multiplexed digital bit stream, using the well-established 8 bit PCM digitization extended to give, for example, an 80 kb/s (Kilobits per second) bit stream. On reception at a receiver, either at a terminal or at a system node or switch, it is necessary to determine bit, frame and multiframe timing. To check synchronism, a synchronizing pattern is sent within the multiframe, which usually embraces four ordinary frames. It is desirable to rapidly synchronize on switch-on and to rapidly resynchronize during receiving from synchronization failure.
One proposed method is to initially send a synchronization signal using the full baseband bandwidth until synchronization is attained, and then revert to the normal synchronization pattern in a multiframe time slot. This has disadvantages due to the need to switch from one synchronization system to another. A second proposed method is to send the normal synchronization pattern in the multiframe, but with all other bits at zero during the synchronization acquisition period. This has limitations in certain implementations, especially in noisy conditions which may cause extended time being needed to achieve synchronization.